1. Field Of The Invention
This invention is directed to methods for separating drilling fluid from a mixture of such fluid and lost circulation materials and to apparatuses useful in such methods.
2. Description of Related Art
Often in drilling a wellbore, the circulation of drilling fluid to and then away from the drill bit ceases due to fracturing of the formation through which the wellbore is being drilled. Drilling fluid pumps into the fractured formation rather than being returned to the surface. When circulation is lost because of fracturing of the formation, it is usually supposed that the fracture occurred at some specific depth where the formation is xe2x80x9cweakxe2x80x9d, and that the fracture extends horizontally away from the borehole. Expressions used to describe rocks that are susceptible to lost returns include terms like vugular limestone, unconsolidated sand, xe2x80x9crottenxe2x80x9d shale, and the like. Whether fractures induced by excessive mud pressure are parallel to the axis of the borehole (vertical) or perpendicular to the axis of the borehole (horizontal) is a subject of some controversy.
To fill or seal off a wellbore fracture so that a proper route for drilling fluid circulation is re-established, a wide variety of xe2x80x9clost circulation materialsxe2x80x9d have been pumped into wellbores. For purposes of classification, some lost circulation materials can generally be divided into fibers, flakes, granules, and mixtures.
The choice of lost circulation material to use in a given case is influenced to some degree by cost and availability in a given drilling area. Cottonseed hulls, for example, are used widely in areas where cotton is grown and drilling in the same area involves mud loss to permeable and cavernous formations. Sawdust is used in areas where lumber is manufactured. Coarse granular material xc2xc inch or xc2xd inch walnut or pecan shells), coarse fiber (shredded hard wood or cedar), medium fiber (shredded redwood or sugar cane), fine fiber (leather, flax, nylon, asbestos) and coarse flake (1 inch cellophane flake) have also been used.
With the bridging agents available today which can be applied through the mud pumps, mud losses to natural and induced fractures, up to xc2xc inch in width, can be plugged. Beer bottles have been successfully applied to a severe loss zone as bridging agents directly down a hole. Rigid hollow objects filled with drilling fluid or a lighter liquid would be strong enough and yet have a density near that of the mud, making it possible for the flow of mud to carry them intact to restrictions in the loss zone. Granular lost circulation material also includes coarse walnut or almond shellsxe2x80x94up to xc2xc inch to xc2xd inch in size; coarse-to-medium wood or cane fiber, medium-to-fine fiber, e.g. wood, cane, nylon, leather; and large cellophane flakes.
The evaluation of lost circulation materials is based on performance tests. In these tests, mud containing lost circulation material is applied under pressure to a simulated formation. Observations are made as to the efficiency of the seal formed at or in a fracture at various concentrations of additive, and the volume of mud lost before a seal is effected. Fibers and flakes have been found to be effective for stopping loss in a highly permeable type of formation, and FIBERTEX(trademark), HYSEAL, and JELFLAKE(trademark) materials that are commercially available are in general use for the same type of loss in the field. Other known lost circulation materials include mica, cellophane, perlite, bagasse fiber, nut shells, feathers, textile fiber blend, and granular materials. Granular materials are more effective than fibers or flakes in some instances for sealing fractures at high pressure. Often the width of fracture that can be sealed depends upon concentration, as well as type, of sealing material. Accordingly, WALL-NUT(trademark) material is used routinely for combating loss of weighted mud because weighted mud tends to induce fracturing of a formation. A general purpose lost circulation material may be characterized by the following criteria: it should contain high-strength granules with a definite size distribution; it should form a seal at both high and low differential pressures; and it should be equally effective in sealing unconsolidated formations and fractures or voids in hard formations.
In one prior art material a mixture of fibers, flakes and granules called KWIK-SEAL(trademark) material is used. Often high filtrate squeezes for lost circulation depend upon tightly packed and substantially dehydrated solids to effect a required seal. This approach to combatting lost returns may be used in either high pressured or normally pressured drilling areas. Other prior art lost circulation materials are commercially available FLOSAL(trademark), HY-SEAL, ZEOGEL(trademark) materials (in proper concentration). In some methods lost circulation materials are mostly granular when drilling with heavy mud, and mostly fibers and flakes when drilling with low-density mud.
A great variety of materials, mixtures and formulas that are pumpable at the surface and develop shear-strength when pumped into place downhole have been used for curing lost circulation. Often an amount of such a material pumped into a wellbore is referred to as a xe2x80x9cplugxe2x80x9d. The plug may develop a xe2x80x9crubbery gelxe2x80x9d or a xe2x80x9cputty-like consistency,xe2x80x9d and breathexe2x80x9d as varied pressures are imposed on natural and induced fractures in the formation. Some as a class have acquired the generic label of xe2x80x9cgunk.xe2x80x9d
One common plug uses bentonite and diesel oil as a base. With certain lost circulation materials, cement and polymers have been added as refinements for some applications. Various ingredients for such a plug are referred to as: DOB=diesel oil bentonite; M-DOB=mud-diesel oil bentonite; DOBC=diesel oil bentonite cement; and M-DOBC=mud-diesel oil bentonite cement. In certain applications diesel oil is used as spacer between gunk and mud or water. The DOB or DOBC slurry is pumped to the bottom of the drill pipe (which is placed somewhat above the loss zone, or at the bottom of the last casing), rams are closed, and the gunk followed by water is squeezed into the formation, or mud is pumped from the annulus as gunk is pumped from the drill pipe and the mixture is squeezed into the formation. Various polymers have been substituted for part of the bentonite in the gunk formula with the hope that the xe2x80x9crubberinessxe2x80x9d of the gel and the xe2x80x9cbreathabilityxe2x80x9d of the plug will thus be enhanced.
When circulation is lost while drilling with oil mud, the same type of squeeze can be applied using water as the continuous liquid, with GELTONE(trademark) commercially available material instead of bentonite as the critical solid in the slurry. High-shear strength is imparted to the slurry when the GELTONE becomes wetted with oil.
Typical known shale shaker screens or screen assemblies with square mesh openings often are clogged or plugged when attempts are made to separate lost circulation materials from a mixture of them with fluid that has been pumped down a wellbore. Stringy, fibrous, and/or fibril material (xe2x80x9cfibrousxe2x80x9d material) can wrap around a wire of a screen and/or bridge a mesh opening without passing through the screen. Although the prior art discloses the use of screens with non-square openings for use on shale shakers for treating mixtures of drilling fluid and drilling solids, the present inventors are unaware of the use of prior art screen(s) and/or screen assemblies with non-square mesh openings in methods for separating fluid from fibrous lost circulation materials.
Various woven cloth screens for vibratory separators are used in removing particles from a liquid and are designed to provide a tortuous path for the liquid. Many prior art woven cloths, including the typical weave, twill, dutch weave or twill dutch weave cloths have low fluid conductance characteristics due to the formation of the tortuous flow path. A minimal rate of flow results in a correspondingly slow filtering process. Often the screens need frequent cleaning to maintain a desired flow rate.
Certain prior art screens provide an open surface area and permit direct or nontortuous flow through the screen. Such screens may provide better conductance characteristics, but the fluid conductance may be limited by the permissible ratio of length to width in the interstices between the screen filaments and the fineness of the filaments. With increasing spacing between filaments, deformation of the filaments from the parallel may increase and larger than desired particles can then pass through the screen. To maintain an efficient relationship, the size of the rectangular interstices in these screens is generally minimal and the length to width ratio is generally less than three unless coarse, stiff filaments are used. Higher ratios can be achieved by bonding together the crossing filaments of the screen; but bonding can be a complex and costly process with negative side effects. By coating the filaments with a bonding material, the diameter of the filaments is increased, further reducing fluid conductance of the screen.
Particle separation, fluid throughput or conductance and screen life are important characteristics of screens for vibratory separators. Finer particle separation results in a higher percentage of impurities being removed from the screened fluid. Higher conductances are desirable because more fluid can be processed per square foot of screen area, thereby reducing costs. Doubling conductance doubles the liquid throughput. Longer screen life saves time and money. Since the mid-seventies one vibrating screen industry trend has been to decrease wire diameter in order to achieve higher conductance. For certain prior art screens this has means finer separation and higher conductance but shorter screen life. To increase screen life, the industry has tried various types of bonded screens such as plastic-backed, metal-backed or bonded-backup. These bonded screens are relatively expensive. U.S. Pat. Nos. 5,370,797; 5,256,291; and 5,256,292 disclose double shute or warp screens with a double warp plain weave screen having warp and shute wires of the same material and properties, the shute diameter at least 1.4 times the warp diameter to prevent sleaziness. If the shute diameter controls the conductance and if the shute diameter is fine enough to give very high conductance, the warp diameter is so fine that the screen has a low tensile strength and therefore shorter life; and screens for removing undesirable particles from a liquid in which a substantially flat parallel array of shute filaments are spaced at less than a preselected minimal linear dimension of undesirable particles and a parallel array of groups of warp filaments runs transverse to the shute filaments. The warp filaments of each group are oppositely woven about and between the shute filaments taken individually or in pairs to secure the shute filaments and maintain the spaces therebetween. The groups of shute filaments have spaces therebetween smaller than the preselected minimal linear dimension of the undesirable particles so that the screen is characterized by elongated rectangular flow apertures therethrough. Each group includes from 3 to 10 or more warp filaments and the shute filament diameters are as small as in the order of 1.1 times the warp filament diameter. Conductance is increased by making the rectangular apertures longer. The life of the screen is increased by increasing the number of warp wires to achieve the required tensile strength. Finer particle separation is achieved by making the short dimension of the rectangle smaller. Screens formed by this weaving of groups of three or more warp filaments transverse to shute filaments which are as small as in the order of 1.1 times the diameter of the warp filaments provide meshes having relatively higher aspect ratios with smaller filament diameters than with certain known weaves of filaments of this range of diameter.
FIGS. 1A and 1B show a prior art screen 22 as disclosed in U.S. Pat. No. 2,723,032 with a coarse mesh wire screen, or cloth 23 that provides a backing screen or cloth of the unit. A fine mesh wire screen 24 is superimposed or mounted upon the backing screen 23. The screen unit 22 has its coarse backing wire mesh or cloth coated or covered preferably with rubber or some suitable rubber or synthetic rubber composition. The strands are indicated at 25 and the covering or coating at 26. Since all of the strands 23 are coated or covered, there is, of course, rubber-to-rubber contact between these strands of the coarser mesh screen 23. The backing screen of cloth 23 is of the roller flat-top type and of any coarse size such, for example, as three of four mesh. The mesh of the finer mesh wire screen 24 varies, in accordance with the separating job to be done. For example, the mesh of the fine wire screen or cloth 24 may vary from the order of minus 20 (xe2x88x9220) to the order of minus 325 (xe2x88x92325).
FIGS. 2A and 2B disclose a screen 30 as disclosed in U.S. Pat. No. 4,696,751 with a first mesh screen with rectangular dimensions of width and length. A second screen 38 is held in superimposed abutting relationship to the first screen 32. The second 38 has width and length dimensions. The length dimensions of the first screen is larger than length dimension of the second screen, and the width dimension of the first screen is smaller than the width dimension of the second screen.
FIGS. 3A and 3B disclose screens 50 and 53 shown in U.S. Pat. No. 5,626,234 which has an upper cloth 51 and lower cloth 52. The upper cloth 51 is formed from woven stainless steel wire in the range 0.19 mm to 0.036 mm diameter and 60-325 mesh, (i.e. number of strands per inch) while the lower cloth 52 is formed from woven phosphor bronze wire in the range 0.45 mm to 0.19 mm diameter and 20-40 mesh. A screen 53 in FIG. 3B has an upper cloth 54 like the upper cloth 51 (FIG. 3A) and a lower cloth 55 woven from stainless steel wire having a nominal diameter in the range 0.20 to 0.45 mm diameter and typical 30 mesh, and is coated with an epoxy based material, or Molybdenum Disulphide, or Teflon (Registered Trade Mark), to a thickness in the range 5 to 50 microns typically 20 to 40 microns. Multiple passes of the wire through a coating process or through a succession of such processes may be necessary to achieve the desired coating thickness. The wires 57, 58, 59 are shown in cross section to show the outer material coatings 67, 68, 69 (not to scale). The wire 64 is shown with the coating scraped from one end.
There has long been a need for a method for efficiently and effectively separating fluid from a mixture of fluid and fibrous lost circulation materials. There has long been a need, recognized by the present inventors, for such a method that does not result in totally clogged or plugged screen assemblies used for such separating.
The present invention, in certain aspects, discloses a method for using a vibratory separator with one or more screen assemblies to separate fluid from a mixture of such fluid and fibrous lost circulation material that is pumped down a wellbore in an effort to remedy a lost circulation problem. In one aspect such a method employs a screen or screen assembly that has at least one layer of screen mesh that has non-square openings, e.g. but not limited to non-square rectangular openings. Fluid flow across such a screen or screen assembly may be in the lengthwise direction of the non-square openings or transverse to the length. It is within the scope of the present invention to use any suitable known vibratory separation apparatus or shale shaker with one or more screen assemblies according to the present invention in methods according to the present invention.
In one particular aspect such a method employs a screen assembly with a lowermost screen of relatively large mesh, e.g. between 15 and 50 mesh; a middle screen of between 105xc3x9764 and 170xc3x97105 mesh (i.e. 105 openings in one direction, 64 openings in the other; 170 openings in one direction 105 openings in the other direction) with openings that are non-square rectangular openings between 333.4 and 178.4 microns long and between 198.7 and 106 microns wide; and a top mesh between 240xc3x97150 and 170xc3x97105 mesh with non-square rectangular openings that are about 136.3 to 72.8 microns wide and 198.7 to 106 microns long. Wire between about 0.016 to 0.0045 inches in diameter is used for the lowermost screen; between 0.0014 to 0.0025 inches in diameter for the middle screen; and between 0.0012 and 0.0018 inches in diameter for the top screen. Alternatively any screen pattern or weave with any wires disclosed herein may be used.
In certain aspects by using non-square rectangular openings a larger opening area is presented to a fiber than is presented by a square opening with a side equal to the width of the rectangular opening (i.e. the length of the non-square rectangular opening is longer than the length of the side of the square). A fiber caught on a wire and/or bridging such a non-square rectangular opening while connected to one or two wires or laying across two wires of such an opening, does not block flow through the non-square opening to the extent that such a fiber would block flow through the square opening, i.e. the percentage of area of the non-square opening blocked by the fiber is less than the percentage of the total area of the square opening that would be blocked by the fiber. In certain aspects in such a situation there may also be more fluid f low against such a fiber and past the fiber""s surfaces when using the non-square openings as compared to the amount of such fluid flow through a square opening. Thus loosening and/or wash through of the fiber may be facilitated by using the non-square openings. Such facilitation may be further enhanced by flowing the fluid to be treated in the general direction of the length of the non-square rectangular openings.
In certain prior art screens that use square mesh openings fibrous lost circulation material can become trapped between the top two meshes of a screen. This is inhibited or reduced by using screens according to the present invention with non-square openings as described above.
It is contrary to the accepted teaching and skill in the art to use screens with the relatively large non-square openings to separate fluid from fibrous lost circulation materials.
What follows are some of, but not all, the objects of this invention. In addition to the specific objects stated below for at least certain preferred embodiments of the invention, other objects and purposes will be readily apparent to one of skill in this art who has the benefit of this invention""s teachings and disclosures. It is, therefore, an object of at least certain preferred embodiments of the present invention to provide:
New, useful, unique, efficient, non-obvious methods for separating fluid from a mixture of fluid and fibrous lost circulating material;
Such methods in which the clogging or plugging of screens of a vibratory separator is inhibited;
Such methods in which screens with non-square openings, e.g. rectangular openings, are used;
Such methods in which fluid flow is in the general direction of the length of the non-square openings; and
Such methods in which a multi-screen screen assembly is used.
The present invention, in certain aspects, discloses a screen with wires of varying diameter in a single screen cloth layer. In one aspect, alternate wires (e.g. but not limited to, every other wire, every third wire, every fourth wire, or every fifth wire) in a screen warp direction are of a larger diameter than the other wires of the screen. In other aspects, alternate wires (e.g. but not limited to, every other wire, every third wire, every fourth wire, or every fifth wire) in a screen shute direction are of a larger diameter than the other wires of the screen. Alternatively, all wires in a warp direction or in a shute direction may be of the larger diameter.
It is, therefore, an object of at least certain preferred embodiments of the present invention to provide:
New, useful, unique, efficient, nonobvious methods for screening lost circulation material with vibratory separators; and
New, useful, unique, efficient, nonobvious vibratory separators (e.g. shale shakers) with such screens.
Certain embodiments of this invention are not limited to any particular individual feature disclosed here, but include combinations of them distinguished from the prior art in their structures and functions. Features of the invention have been broadly described so that the detailed descriptions that follow may be better understood, and in order that the contributions of this invention to the arts may be better appreciated. There are, of course, additional aspects of the invention described below and which may be included in the subject matter of the claims to this invention. Those skilled in the art who have the benefit of this invention, its teachings, and suggestions will appreciate that the conceptions of this disclosure may be used as a creative basis for designing other structures, methods and systems for carrying out and practicing the present invention. The claims of this invention are to be read to include any legally equivalent devices or methods which do not depart from the spirit and scope of the present invention.
The present invention recognizes and addresses the previously-mentioned problems and long-felt needs and provides a solution to those problems and a satisfactory meeting of those needs in its various possible embodiments and equivalents thereof. To one skilled in this art who has the benefits of this invention""s realizations, teachings, disclosures, and suggestions, other purposes and advantages will be appreciated from the following description of preferred embodiments, given for the purpose of disclosure, when taken in conjunction with the accompanying drawings. The detail in these descriptions is not intended to thwart this patent""s object to claim this invention no matter how others may later disguise it by variations in form or additions of further improvements.